May 17, 2007
Nanospears strike at gas detection
Spear-like In-O-N nanostructures created in China could be ideal for gas sensing applications such as NO2 and alcohol detection as well as the measurement of air humidity. Thanks to their high surface area, nanospears have the potential to outperform simpler structures including nanowires, nanobelts and nanotubes say scientists.
"A nanospear offers a fourfold symmetrical structure, a diameter change along the growth axis and a larger morphology surface at the tip as it grows from a nanowire into a nanospear," Baoyu Liu of Liaoning University of Petroleum and Chemical Technology told nanotechweb.org. "The difference in diameter, morphology surface and shape of the tip and the shaft will modify the nanospear's properties and functionality."
Analysis revealed that the tip of the spear has a higher nitrogen content compared with the shaft. According to the researchers, this can be explained by the tip's larger surface area and its subsequent ability to absorb more nitrogen during synthesis.
Together with colleagues from Jiangsu and Zhejiang universities, Liu creates the structures using a simple thermal evaporation process that involves ammoniating InP powder at temperatures of up to 1100°C. Firstly, a Al2O3 boat containing the powder is heated to 800°C under the protection of argon gas (Ar) with a flow rate of 200 ml/min. Next, ammonia (NH3) is introduced at stable flow rates between 50 and 400 ml/min to replace the Ar and the furnace temperature is increased. The resulting nanostructures are deposited on a piece of silicon wafer placed downstream of the reaction chamber.
"The original objective of this experiment was to grow InN nanostructured materials by reacting InP powder with NH3 gas," she revealed. "When we analysed the product, we discovered a few of the spear-like In-O-N nanowires mixed in with the InN particles."
By tuning the gas flow rate, the team has managed to dramatically increase the production of its nanospears. It turns out that higher NH3 flow rates are key to promoting the growth of the fourfold symmetrical structures, but even under these conditions InN particles remain in the final product. Going forwards, the group needs to find a way of excluding the InN structures to drive up the purity of its In-O-N output.
The researchers reported their work in Nanotechnology.
About the author
James Tyrrell is editor of nanotechweb.org